1. Field of the Invention
The present invention relates to a nonvolatile semiconductor memory device.
2. Description of the Related Art
As a nonvolatile semiconductor memory device of an irreversible type, which retains stored information even when power is disconnected, there is a one time programmable (OTP) memory to which data can be written only once.
Examples of a memory element used in an electrically writable OTP memory include an anti-fuse element of a MOS structure or a capacitor structure. Writing of data to the OTP memory is performed by breaking down or not breaking down an insulating film between both terminals of the anti-fuse element.
Specifically, when the insulating film between both terminals of an anti-fuse element is broken down by application of a high voltage between both the terminals of the anti-fuse element, the anti-fuse element changes to a low resistance state. Accordingly, when a voltage is applied between both the terminals, the current flows through the anti-fuse element. Here, this state is referred to as a retention state of “1” data.
On the other hand, when no voltage is applied between both the terminals of the anti-fuse element, the insulating film of the anti-fuse element is not broken down, so that no current flows through the anti-fuse element. In this respect, this state is referred to as a retention state of “0” data.
A read operation of such an OTP memory is performed by applying, between both the terminals of the anti-fuse element, a voltage low enough not to break down the insulating film, and then detecting whether a potential of a bit line connected to the anti-fuse element increases. If the insulating film is broken down, the current flows through the anti-fuse element, so that the potential of the bit line increases. In other words, “1” data is read out.
As described above, the operations to write and read data to an anti-fuse element are performed in the OTP memory by the simple operation, which is only to apply a voltage between both the terminals of the anti-fuse element. For this reason, the field of application of the OTP memory has been spreading to cover applications for redundancy of a large capacity memory, tuning of an analog circuit, storing of a code such as a cryptography key, management of a manufacturing history and the like. Thus, use of the OPT memory in the future as an irreversible nonvolatile semiconductor memory device is most expected to grow.
However, when the operation to write “1” data is performed, breaking down of the insulating film alone as in the case of the aforementioned OTP memory does not change the anti-fuse element to a sufficiently low resistance state, so that it is necessary to flow a large current after the breaking down of the insulating film. Accordingly, the current capacity of a circuit to generate a high voltage for breaking down the insulating film needs to be increased. However, since the current capacity is limited, the number of bits to which data can be written simultaneously is restricted.
In order to perform writing to multiple bits simultaneously under such a restriction, there has been proposed heretofore a method in which a high voltage for writing is repeatedly applied to all bits of the same word line, and the writing is performed several times in a separated manner (refer to Japanese Patent Application Publication No. 2006-338766 (pages 7 to 8, FIG. 1)).
However, in the aforementioned proposed method, the high voltage applied at the time of later writing is also applied to the anti-fuse element whose insulating film has been broken down and has thus become a low resistance state earlier. Accordingly, a large current flows through the anti-fuse element whose insulating film has been broken down, so that a lack of the current capacity occurs in the circuit generating the high voltage for breaking down the insulating films. As a result, there arises a concern that the sufficient amount of the write voltage and the large current flowing through after the break down of the insulating film may not be obtained. In other words, there arises a problem that writing of data cannot be performed stably.
Moreover, in the aforementioned OTP memory, a current flows through an anti-fuse element when a read operation is performed. Thus, the amount of current consumption at the time of the read operation is desirably reduced.